Pulse-amplitude modulation (PAM) signaling may be modulated using QAM (quadrature amplitude modulation) or VSB (vestigial sideband) modulation, which are well known to one having ordinary skill in the art. These and other PAM related modulation techniques include converting an input data stream into symbols that are then filtered, modulated and mixed to a desired frequency for transmission. The filtering may be used to limit the corresponding signaling bandwidth and/or to prevent inter-symbol interference (ISI). Examples of QAM signals are QPSK (quadrature phase shift keying), and N-QAM, where N is the number of points in a constellation. As the number N increases, each symbol may carry more information, or bits of data. If transmission impairments, such as high random noise or brief bursts of impulse energy are anticipated, a forward error correcting code can be applied to the transmission (this may result in more symbols being transmitted). The data of a PAM transmissions may be transmitted continuously, such as in broadcast signals or in blocks.
PAM signals may be contaminated with linear distortions in a signal path during transmission, such as from echoes, group delay and other factors. Linear distortions create inter-symbol interference, which can cause errors in the transmission, or in severe cases, cause the transmission to be unintelligible. Linear distortions may be removed by an adaptive equalizer structure, such as a FIR (finite impulse response) filter, or IIR (infinite impulse response filter). The adaptive equalizer may be located at the receiver or at the transmitter and/or a transceiver may include capabilities to facilitate equalizing in both receiving and transmitting directions. If the equalizer is located at the transmitter it must have knowledge of the linear distortion in the transmission path, and the technique is called “pre-distortion”.
OFDM (orthogonal frequency division multiplexing) block transmissions are also well-known in the art. With OFDM block transmissions, the symbols to be transmitted are converted with an IFFT (inverse fast Fourier transform). This creates component subcarriers from symbols. Received OFDM signals may be comprised of a single signal from a single transmitter, or by a composite signal comprised of OFDM signals from multiple simultaneous transmitters. In the latter case the modulation technique is known as orthogonal frequency division multiple access (OFDMA). OFDM transmissions are sent in individual blocks, or streams of blocks that may be contiguous. OFDM signals may also become contaminated by linear distortions.
A guard interval (a.k.a. cyclic prefix) may then be added to OFDM and PAM transmissions by copying symbols from the end of the block transmission and pasting them on to the front of the block transmission. If the guard interval is longer than the longest echo in the channel, the effects of the echo can be removed at the receiver using frequency domain equalization. OFDM signals may be equalized at the receiver by a single complex multiplication of each subcarrier by a correction number (typically complex), provided the duration of the echo is shorter than the duration of the guard interval. At the receiver, the PAM block transmission may be converted into the frequency domain where linear distortion is removed with a single complex multiplication on each subcarrier.
OFDM and QAM signals, being very different in how they are created, have different characteristics for transmission. OFDM signals have a disadvantage of a high crest factor (peak to average power ratio), but have an advantage that the loss of a few component subcarriers in the frequency domain can be tolerated if component subcarriers are coded using a FEC (forward error correction). QAM signals have an advantage of a lower crest factor, and can tolerate a short duration disruptive temporal noise burst if a forward error correcting code is used. Thus, using FEC, OFDM can tolerate some missing component subcarriers (which can be caused by an impairment in the frequency domain, such as an interfering carrier), and QAM can tolerate some missing symbols (which can be caused by an impairment in the time domain, such as a short burst of noise).
A cable operators utilizing hybrid fiber coax (HFC) plant may desire extending the digital transmission capability of their plant. One option contemplated by the present invention is to change a frequency split from a 5-42 MHz upstream and 54-860 MHz downstream split to a 5-200 MHz upstream and 250-860 MHz downstream split and another contemplated option is to use bandwidth above 900 MHz for upstream and/or downstream signals.
A wide, high-bandwidth carrier is desirable for transmitting signals because it can provide a simpler MAC (media access control) layer. A wide bandwidth PAM transmission has a very short symbol duration, which creates a problem with a FIR (finite impulse response) equalization circuit in this type of option. For example, an echo may be 2 microseconds in duration, but the symbol rate could be as short as 5-10 nanoseconds. With PAM, this would require a FIR filter structure with more than 600 taps (digital filter) running with a high clock rate, which may not cost be effective. (A digital filter tap may be distinguished from a plant tap. The digital filter taps are part of a digital filter and may are implemented as gates in a silicon chip whereas the plant taps may be a physical device connected to the coax with 4 ports (typ.))
The present invention contemplates the use of OFDM as a solution to this problem. Unfortunately, OFDM has a high crest factor, which creates a need for an expensive high-powered transmitter to generate the required transmit power, particularly for the above 900 MHz option where coaxial cable attenuation may be high.
In a tree and branch architecture used by cable, some subscribers will have low cable attenuation, and some will have high cable attenuation. The subscribers with high cable attenuation will require higher transmit power, which may cause the high crest factor of OFDM to be a disadvantage for those subscribers such that PAM is preferred. Subscribers with low cable attenuation can take advantage of the low-noise signal path by using high-order modulation symbols which carry more bits per symbol. Thus, the present invention contemplates a need for a transmission system that can produce a low crest factor signal (PAM), allows simultaneous transmissions (both), has as short a guard interval as the signal path allows (both), has simple efficient equalization, (both) and uses high order modulation if the signal path allows (both). This need applies equally to cable plants, wireless or non-cable plants and other area where the characteristics of transmission mediums (a.k.a. signal paths) used by multiple subscribers varies from subscriber to subscriber